Arsenic is widely distributed in natural water environment. Arsenic normally exists as arsenate (As(V)) in surface water bodies. However, under reductive conditions such as underground water, the dominant species of arsenic is arsenite (As(III)). Arsenic has been reported to show acute and chronic toxicity to human beings, and the long term exposure to arsenic via drinking water has serious effects on the health of human beings.
The technologies for the removal of arsenic from drinking water receive great concern from researchers from all over the world. There are several technologies, i.e., coagulation, precipitation and/or co-precipitation, adsorption, membrane filtration, ion exchange, that have been demonstrated to be feasible. Coagulation is normally adapted in municipal drinking water treatment plants. Membrane filtration may be automatically controlled, but could not achieve satisfied removal of arsenic unless the reverse osmosis (RO) is used. The high cost and the simultaneous removal of large scale co-existent ions during the removal of arsenic also preclude the implementation of RO in practice. As for the ion exchange method, the operation is complicated and the cost is very high. Adsorption shows several advantages such as low cost, simple operation, and high effectiveness, and is viewed as one of the best methods for the removal of arsenic from drinking water in small systems, especially in rural areas.
Adsorption uses insoluble adsorbents with high specific surface areas and fine mechanic strength to facilitate the adsorption of arsenic onto the surfaces of adsorbents, through physical adsorption, chemical adsorption, and/or ion exchange, so as to achieve the removal of arsenic from water. Adsorption is easy to handle, and is especially suitable to the treatment of arsenic-polluted drinking water with relatively large amount and with low arsenic concentrations.
There has reported several common adsorbents of natural coral, bentonite, zeolite, red mud, coconut shell, coated granule, activated aluminum oxides, activated carbon, and natural or synthetic metal oxides and hydroxides. Many adsorbents show good removal capability towards As(V), but limited removal capability towards As(III). The toxicity of As(III) is more than 60 times to that of As(V), and the removal of As(III) is much more difficult than that of As(V). Consequently, the oxidation of As(III) to As(V) by oxidants, which is followed by the adsorption of As(V) onto adsorbents, is often employed in the treatment of arsenic from drinking water. However, the addition of oxidants would inevitably result in the formation of by-products that shows potential side effects to health of human beings, and the use of chemical oxidants should be avoided or minimized in drinking water treatment processes. Additionally, as for the commonly-used adsorbents, those with low cost generally exhibit low adsorbing capability towards arsenic, and the prices of those with high adsorbing capability are often much too high to inhibit its large scale application in practice. Furthermore, the conventional adsorbents are often produced by the coating of components with adsorptive activities onto porous carriers through the procedures such as soaking, precipitation, and sintering. Unfortunately, the amount of active components would inevitably decrease in the long term operation of adsorption units due to collision and friction between adsorbent particles. The complicated regeneration procedures including soaking, precipitation, and sintering again, are required to retrieve the adsorption activity, which highly precludes its application in engineering. Consequently, it is difficult to develop novel adsorbents with advantages of economical efficiency, easy-to-handle, easy-to-regeneration, and high effectiveness towards both As(III) and As(V).
The present invention relates to a ferric and manganese binary oxide (FMBO) based adsorbent, and the technological principle of removing arsenic includes: 1) during the preparation of the adsorbent, the manganese oxides (Mn(IV) oxides) with oxidative capability and the iron oxides with adsorptive capability are precipitated and in situ coated on the surfaces of porous carriers through the in situ coating procedures; 2) during the adsorption of arsenic, the Mn(IV) oxides contributes to the transformation of As(III) to As(V) through catalytic oxidation effects, and the reductive dissolution of Mn(IV) oxides to Mn(II) increases the adsorptive sites (i.e., surface hydroxyl) and significantly enhances the capability of removing arsenic; 3) during the regeneration of the adsorbent with adsorbed arsenic on its surfaces, the active component is in situ coated onto the surfaces with another layer of FMBO covering the surfaces, and the adsorbed arsenic is solidified in the adsorbent to simplify the regeneration procedures and to avoid the production of high-arsenic wastewater during regeneration. The present invention overcomes many disadvantages stated above, and may simultaneously remove As(III) and As(V) from drinking water to meet the MCLs level of arsenic being lower than 10 μg·L−1, as being required by the World Health Organization, U.S. Environmental Agency (U.S.EPA), and the National Drinking Water Standard of China No. GB5749-2006.